1. Field of the Invention
The present invention relates to frequency measurement and, more particularly, to frequency measurements of various quartz crystal resonant modes.
2. Description of the Prior Art
It is often desirable to determine the resonant modes of a quartz crystal arrangement where the frequency information may be used in scientific study, for example. Frequency characteristics of interest may include resonant frequencies, inharmonic overtones, and harmonic modes of the crystal arrangement.
Historically, methods of measuring resonant modes of a crystal include swept frequency sinusoidal analysis and crystal oscillator testing. The swept frequency sinusoidal analysis involves applying a sinusoidal signal to the crystal arrangement and sweeping the frequency of the sinusoidal input signal through a frequency range of interest. Based on the response of the crystal arrangement to the sinusoidal input, certain frequency characteristics can be obtained. The swept frequency sinusoidal analysis, however, is often tedious and overly time consuming, and often requires repetitive cycles of: providing an input signal to the crystal arrangement, changing the frequency of the input signal, measurement of the response of the crystal arrangement, comparison of the gathered information, and determining the input signal to be applied to the crystal arrangement for a subsequent measurement based on the current cycle measurement. Typically, for a high accuracy measurement very precise and expensive measurement equipment must be used. However, even when measurements are taken with the very precise and expensive equipment, errors may still arise due to human intervention during the analysis.
The crystal oscillator testing method for measuring a resonant mode of a crystal involves placing the crystal in an electronic oscillator circuit and measuring the oscillator signal frequency. Due to the resonant properties of a crystal, the electronic oscillator circuit will converge to the crystal's resonant frequency. While crystal oscillator testing is a reasonable method to determine the principal resonant frequency of a crystal, it is an undesirable method where additional resonant modes are to be measured.